Add samples for iterator-related extensions (KT-20357)

libraries/stdlib/src/kotlin/collections/Iterators.kt

@@ -5,6 +5,7 @@ package kotlin.collections
 
 /**
  * Creates an [Iterator] for an [Enumeration], allowing to use it in `for` loops.
+ * @sample samples.collections.Iterators.iteratorForEnumeration
  */
 @kotlin.jvm.JvmVersion
 public operator fun <T> java.util.Enumeration<T>.iterator(): Iterator<T> = object : Iterator<T> {
@@ -15,6 +16,7 @@ public operator fun <T> java.util.Enumeration<T>.iterator(): Iterator<T> = objec
 
 /**
  * Returns the given iterator itself. This allows to use an instance of iterator in a `for` loop.
+ * @sample samples.collections.Iterators.iterator
  */
 @kotlin.internal.InlineOnly
 public inline operator fun <T> Iterator<T>.iterator(): Iterator<T> = this
@@ -22,11 +24,13 @@ public inline operator fun <T> Iterator<T>.iterator(): Iterator<T> = this
 /**
  * Returns an [Iterator] wrapping each value produced by this [Iterator] with the [IndexedValue],
  * containing value and it's index.
+ * @sample samples.collections.Iterators.withIndexIterator
  */
 public fun <T> Iterator<T>.withIndex(): Iterator<IndexedValue<T>> = IndexingIterator(this)
 
 /**
  * Performs the given [operation] on each element of this [Iterator].
+ * @sample samples.collections.Iterators.forEachIterator
  */
 public inline fun <T> Iterator<T>.forEach(operation: (T) -> Unit) : Unit {
     for (element in this) operation(element)